A prior art noise reduction system is disclosed, for example, in U.S. Pat. No. 4,064,530. In the disclosed system, in order to improve the S/N ratio, one to several frame portions of a video signal are recursively added to the present frame so long as there is a high inter-frame correlation as, for example, in the case of a still object being shown. When the inter-frame correlation is relatively low, as, for example, in the case of a fast moving object being shown, either no addition is performed or fewer frame portions are recursively added to the present frame. This is done in order to prevent the formation of an afterimage or lag that unavoidably accompanies recursive frame-to-frame addition for noise reduction.
It is known that the degree of viewer offense to noise in a displayed television picture depends on the level of the input video signal. If the object shown is sufficiently bright so as to make the level of the incoming video signal sufficiently high, noise is not recognized. Noise reduction therefore provides a very limited effect under such situations, as is reported in a paper titled "PERCEPTION OF RANDOM MONOCHROME VIDEO INTERFERENCE" NHK Gijutsu Kenkyu (NHK Technical Journal - in Japanese), September 1958 Vol. 10, No. 5, page 18 to page 24. Considerating this factual data with reference to a high-level input video signal, it can be seen that noise reduction is achieved with minimum effort.
On the other hand, experimental data collected by the inventors of the present invention, indicate that a high-level input video signal, when subjected to the same degree of noise reduction as that for a low-level signal, makes the reproduced picture of a human face appear keloidal.
It is also known that most noise components are in the higher frequency region of a television video signal and that they cause interference in the chrominance component of video signals. If the level of the chrominance component is high, such noise can cause blurring and smear along the profile of objects on the reproduced display. According to a conventional noise reduction system, if the effect of noise reduction is sufficiently enhanced to remove such unwanted noise, signal components in the lower frequency region are equally subjected to the increased noise reduction effect, producing a more pronounced lag or afterimage.
Therefore, to eliminate the defects in a conventional noise reducing technique, the noise reduction effect must be controlled in response not only to the degree of interframe correlation but also to the level of noise contained in the input video signal. This is required as signal processing for noise reduction, applied to a high-level video signal, results only in adverse effects rather than any appreciable improvement in the S/N ratio.
It is therefore one object of this invention to provide a noise reduction system which is capable of controlling the effects of noise reduction in response to the level of an input video signal to thereby eliminate the visually adverse effect encountered with conventional techniques.
Another object of this invention is to provide a noise reduction system which is capable of improving the S/N ratio for the high frequency region of a video signal.
Still another object of this invention is to provide a lag-free noise reduction system, which minimizes the possibilities of interference with the chrominance component, by emphasizing the noise reduction effects for high frequency regions of the video signal, with the frequency characteristics of the noise contained in the video signal being considered.
A further object of this invention is to provide a noise reduction system capable of controlling the degree of the noise reduction effect depending on the level of noise contained in the input video signal.